Traveling wave tube having tapered grooves and shims for improved thermal contact between metal envelope, support rods and slow wave helix



3,374,388 ND SHIMS FOR OPE, SUPPORT R, E. HUBER NG WAVE TUBE HAVINGTAPERED GR D THERMAL CONTACT BETWEEN META RODS March 19, 1968 TRAV IMPROOVEJS A L ENVEL LIX WAVE H 15, 196

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RAYMOND E. HUBER j ATTORNEYS United States Patent 3,374,388 TRAVELINGWAVE TUBE HAVHNG TAPERED GROOVES AND SHiMS FOR IMPROVED THERMAL CONTACTBETWEEN METAL EN- VELQPE, SUPPORT RODS AND SLOW WAVE HELIX Raymond E.Huber, Woodbine, Md., assignor, by mesne assignments, to the UnitedStates of America as represented by the Secretary of the Navy Filed Nov.13, 1964, Ser. No. 411,169 Claims. (Cl. 3153.5)

ABSTRACT 0F THE DISCLOSURE A traveling wave tube having a tubularenvelope with longitudinally extending grooves of uniformly increasingdepth between their ends spaced about the inner periphery, elongatedtapered shims each contiguous with the radially outermost surface of arespective groove and forming an inner surface parallel to thelongitudinal axis of the envelope, elongated dielectric support rodscontiguous with a respective shim inner surface and forming an innersurface parallel to the longitudinal axis of the envelope, and a helicaldelay winding coaxially supported in the envelope by the rod innersurfaces whereby posi tioning of the shims along the length of theenvelope obtains positive contact between interfaces of the winding,rods, shims and envelope and thereby affords high thermal conductivitytherebetween.

The present invention relates to high frequency electron dischargedevices, and more specifically to traveling wave tubes and slow-wavecircuits therefor.

In a conventional traveling wave tube an electron beam is projectedalong the longitudinal axis of a relatively 7 long, slender, helicalconductor at an appropriate velocity for interaction with high frequencysignal waves traveling along the helix in the same direction, for thepurpose of amplifying the signal. The essential elements of such a tubeare an electron gun, a relatively long, slender, helical conductor ofsmall diameter, a conductor, an envelope enclosing these elements, andmeans for coupling the input and output ends of the helical conductor toinput and output transmission lines. The tube is normally mounted in asuitable magnetic field coil, or a permanent magnet, to provide afocusing magnetic field axial to the electron beam. Usually the helicalconductor of a traveling wave tube is rigidly held in place within theenvelope or tube between and by several ceramic rods circumferentiallypositioned about the outside of the helix. See US. Patent No. 3,026,445to T. H. Stoerck for Traveling Wave Electron Discharge Tubes issued Mar.20, 1962. e

The helix and helix-derived slow-Wave structures employed in travelingwave tubes of the prior art have good bandwidth and impedancecharacteristics at lower power levels of operation. However, whenattempts are made to operate such structures at higher power levelsthermal problems become increasingly pronounced.

Attempts have been made to solve the thermal problem by applying forceon the slow-wave structure and dielectric support rods of traveling wavetubes to assure good physical contact between the metal slow-wavestructure, the support rods, and the metal tube envelope: good physicalcontact is requiredfor high thermal conductivity at the aforesaidinterfaces.

One process now employed to obtain high thermal conductivity involvesthe deformation of the tube envelope to a point slightly below theelastic limit thereof. The slow-wave structure and support rods are theninserted into the now out-of-round envelope. The support rods are placedat the points of maximum envelope width. When the deforming force isremoved the envelope attempts to return to its original cylindricalshape clamping the slow-wave structure and support rods in place. Theforce impressed upon the slow-wave structure and support rods varieswith the differences between the outside diameter of the slow-wavestructure and support rods and the smaller inside diameter of theenvelope.

Another process uses two envelope halves, the inside diameter of whichis smaller than the outside diameter of the slow-wave structure andsupport rods. The slowwave structure and support rods are placed on oneenvelope half and the other half is placed on this assembly. This unitis then fixtured to close the gap between the body halves and brazed toseal the halves together. The force exerted on the slow-wave structureand support rods varies with the difference in diameters as in theprocess discussed above.

The disadvantage of the above-mentioned processes is the required highdegree of accuracy in fabrication necessary to obtain the desireddiameter difference and the desired force thereby impressed upon theslow-wave structure. Also, the prior art methods exhibit a lack ofcontrol of the concentricity of the slow-wave structure with respect tothe inside diameter of the envelope. Furthermore, the prior art methodsexhibit little control over the amount of force applied to the slow-wavestructure and support rods other than that force developed due to theinitial difference in diameters discussed above. It is readily observedthat in the first process there is a risk of exceeding the elastic limitof the metallic tube envelope while deforming it to accept the slow-wavestructure and support rods. In the second process there is a requirementfor elaborate fixturing or machining to maintain alignment while brazingthe body halves together, and during this process there is thepossibility of decreasing the amount of attenuation on the dielectricsupport rods due to furnace brazing the body halves together in ahydrogen atmosphere.

The present invention solves the problem of applying suflicient force tothe slow wave structure and dielectric support rods of traveling wavetubes and assures good physical contact between the metal slow-wavestructure, the support rods, and the metal tube envelope, therebyobtaining high thermal conductivity at the aforesaid interfaces. This isobtained by employing an envelope, the inside diameter of which tapersfrom one end to the other or alternatively an envelope with grooveswhich'taper from one end to the other and metal shims of the same taperangle but of greater length. The shims are placed between the supportrods of the slow-wave structure and the tube envelope with the tapersopposing each other'to present parallel surfaces to the support rods.

It is an object of the present invention to provide a loading mechanismfor a traveling wave tube slow-wave structure for providing high thermalconductivity.

Another object of the present invention is to provide a loadingmechanism for a traveling Wave tube slow-wave structure which enablesthe control of the concentricity of the slow-wave structure with respectto the inside diameter of an enclosing envelope.

It is a further object of the present invention to provide a loadingmechanism for a traveling wave tube slow-wave structure which permitseasy control over the amount of force applied to the slow-wave structureand support rods.

A still further object of the present invention is to provide a loadingmechanism for a traveling wave tube slowwave structure which providesfacile control over the amount of force applied to the slow-wavestructure and support rods.

. .Stillanotherobject of the present invention is to pro vide a loadingmechanism for a traveling wave tube slowwave structure for adjusting theattenuation characteristiqs of the traveling wave tube as well as foradjusting the phase of the velocity characteristics of the slow-wavestructure.

A still further object of thepresent invention is to provide .a loadingmechanism fora traveling wave tube slow-wave structure, ,which providescontrol over the amount of force exerted on the slow-wave structure andsupport rods, control over the concentricity of the slowwave structurewith respect to the, envelope inside diameter, and which, by virtue ofcontrol of the amount offorce exertedonthe slow-wave structure, controlsthe attenuation and phase velocity characteristics of the slowwavestructure. a I

.These. and other objects and features of the invention will becomeapparent to those skilled in the art as the .disclosureis made in thefollowing description of an embodimentof the invention as illustrated inthe ac- .cornpanying drawings which show, so far as is necessary to anunderstanding of the said invention, a traveling wave tube of the kindreferred to and embodying the said invention.

.FIG. 1 is a schematic elevation, broken away at the ends, of. theembodiment in question; and

FIG. 2 is a sectional view taken along the lines 2--2 of FIG. 1..

As illustrated in the drawings, the traveling wave tube .10, partlyrrepresented, has within a cylindrical metallic envelope 11, shownbroken away, a helical delay winding 12 ,hav ing one end thereofattached to an RF input coupling means, not shown, and having the otherend thereof coupledto an RF output coupling means, also not shown. Thewinding 12 coaxial'ly surrounds an electron discharge path, notrepresented and emanating from an electron gun known and used in theart. The path extends axially and interiorally of the winding 12 in thecustomary manner. The. winding 12 is supported by four insulating rods15 of dielectric materialspaced at 90 intervals about the windingperiphery and contacting it along the periphery thereof. I a

In accordance with this invention the envelope 11 is provided ,with fourgrooves 29 formed within the interior surface ofthe envelope 11 andspaced at 90 intervals about. the inside circumference thereof. Groovesare tapered from the insertion end 110 of envelope 11 to the other end11b of the envelope 11. In other words, the distance 1.,the distancebetween the longitudinal central .axis of envelope 11 to the outermostwall 21 of groove 20, decreases with increased distance from insertionend 11a;

.- ."Dielectric support rods 15 are of constant thickness throughout thelongitudinal length thereof and are par- .tiallylseatedwithin grooves20.. To obtain good physical contact necessary for ,high thermalconductivity between the metalslow-wave structure 12, the support rods15 and .the metal tube envelope 11, metal shims are inserted withingroove 20 between each support rod 15 and the tapered wall 21. Eachmetal shim 30 is tapered from one end tothe other at the, same taperangle as groove 20 but is of-greatertlength. Shim 30 includes ahorizontal rodabutment surface 31 and a tapered surface 33, and isinsertedbetween the support rods of the slow-wave structure 12 and thetube envelope 11 with the tapered surfaces33 opposing the taperedsurface 21 of groove 20 to thereby presenttparallel surfaces to thesupport rods.

, As may be. readily seen from the figures, longitudinal movementoftheshim will cause a radial displacement of the support rods 15 andslow-wave structures 12. By

.manipulating opposing shims, concentricity of the slowwave structure 12with respect to. the inside diameter of .the tube envelope 11 may beattained. By manipulating the shims collectively, after concentricityhas been attained, the desired force may be imposed on the slowwavestructure. By knowing the amount of force required for good physicalcontact of the slow-wave structure members and the mechanical advantagederived from the taper angle, the required displacement of the shims toattain the desired force can be calculated. Should good physical contactnecessary for high thermal conductivity be less important thanattentuation characteristics, as in low power traveling wave tubes, theattenuation characteristics may be adjusted slightly by varying theforce imposed on the slow-wave structure 12. The phase velocitycharacteristics of the slow-wave structure may be adjusted in a similarmanner.

It will be further observed that the inward pressure forces applied bythe shims 30 are uniform over the entire length of the slow-Wavestructure 12 so that a very good and stable positioning and clampingaction is obtained.

It will be understood that various changes in the details, materials,steps and arrangement of parts, which have been herein described andillustrated in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims.

, What is claimed is:

1. A traveling wave electronic discharge tube of the type having a delaywinding which is coaxial with the electron beam path of the tube, saidtube including a plurality of rods which support said delay winding andare in contact with the outsidethereof along a plurality of linesextending parallel to the axis and equally spaced aboutthe peripherythereof comprising:

a plurality of grooves equally spaced about the inner circumference ofthe tube and extending longitudinally along the length thereof, each ofsaid grooves having a surface inclined from longitudinal alignment andextending from one of the groove ends to h the other;

and elongatedshim means inserted in said grooves, each said shim meansbeing tapered along the length thereof and having an inclined surfacedefining the taper of said shim means in abutting engagcrnent'with saidinclined surface of a respective said groove whereby parallel surfacesare presented to the support rods.

2. The invention as defined in claim 1 wherein said tapered shim meansis of a length greater than the longitudinal extent of said grooves. I I

3. The invention as defined in claim 1 wherein said shim means isconstructed of thermal conducting material.

4. The invention as defined in claim 1 wherein said shim means isconstructed of metal.

, 5 The invention as defined in claim 1 wherein four grooves areprovided at intervals about the interior circumference ofthe tube,

a 6. The invention as defined in claim 1 wherein each said inclinedsurface defines a respective groove depth progressively decreasing fromone of its respective ends to the other. i

7. The invention as defined in claim 6 wherein said inclined surfaces ofsaid grooves have inclinations relative to the tube axisof thesamemagnitude,

'8. The invention as defined in claim 1 wherein said inclined groovesurfaces diverge relative to the tube axis in the same longitudinaldirection.

9. The invention as defined in claim 8 wherein:

said grooves have rectangular cross-sections; and

said inclined surfaces of said grooves have inclinations relative to thetube axis of the same magnitude.

10. In a high frequency electron discharge device of the type includingan electron gun disposed at one end of a predetermined path forgenerating and directing an electron beam along such predetermined path,a collector structure disposed at the other end of said predeterminedpath for collecting said beam, a slow-wave circuit disposed along saidpredetermined path between said electron gun and said collectorstructure, said slow-wave circuit being adapted and arranged such thatenergy exchange 5 between the electron beam and radio-frequency energypropagating on said slow-wave circuit is achieved, said slow-wavecircuit including a helix disposed around said predetermined path, andfurther including a plurality of rods supporting said helix and incontact with the outside thereof and equally spaced about the peripherythereof, the improvement comprising:

a thermally conductive tubular envelope surrounding the predeterminedpath and including an interior circumferential surface;

a plurality of equally spaced grooves formed in said interiorcircumferential surface and extending parallel to the longitudinal axisof said envelope, each of said grooves having an inclined surfacedefining a groove depth progressively decreasing from one of 1 its endsto the other; and

longitudinally extending shim means each having an inclined surfaceimparting a longitudinal taper thereto and each inserted in a respectiveone of said grooves between said groove and said support rods, each saidshim means being inserted within said respective grooves with saidinclined surfaces opposing each other and presenting parallel surfacesto said support rods.

References Cited UNITED STATES PATENTS 9/1966 Washburn 315-35 HERMANKARL SAALBACH, Primary Examiner. PAUL GENSLER, Examiner.

